1. Field of the Invention
The present invention relates to a semiconductor laser and a method for manufacturing the same. More particularly, the present invention relates to a method for manufacturing a semiconductor laser with excellent heat resistant properties to avoid the deterioration in qualities of a coating film provided at an end face thereof under high temperature conditions, so that a level of Catastrophic Optical Damage (hereinafter abbreviated as COD) can be reduced and reliability can be improved.
2. Related Background Art
In recent years, recording and reproducing type optical disk devices such as DVD-RAM and CD-R are in increasing demand. GaInP/AlGaInP based semiconductor lasers (emission wavelength: 630 to 680 nm) and AlGaAs based semiconductor lasers (emission wavelength: 780 to 800 nm), which are used as pick-up light sources for such devices, are demanded to have higher output power for higher speed and higher reliability for a stable operation over an extended term.
In the above-stated high-power semiconductor lasers, in order to allow a laser beam to emit effectively from an end face of a resonator, normally, a low-reflection end face coating film is formed on one of the end faces of the resonator, whereas a high-reflection end face coating film is formed on the other end face of the resonator. Here, in the case of a semiconductor laser formed by cleavage, the end face of the resonator denotes a crystal face obtained by the cleavage.
The high-reflection end face coating film normally has a multilayer structure made up of two kinds of dielectrics including a dielectric with a high refractive index such as amorphous silicon and a dielectric with a low refractive index such as alumina or silicon oxide, which are laminated alternately with a thickness of λ/4 n normally where λ and n respectively denote the emission wavelength of a laser beam and the refractive index. Also, the reflectance around 95% is required often for the high-reflection end face coating film, so that the film may be configured with two or three sets of the above-described lamination including two kinds of dielectrics laminated alternately. The dielectrics mentioned here include a dielectric having insulating properties.
Meanwhile, in the high-power semiconductor lasers, with an increase in the optical output, recombination without light emission increases in the vicinity of the resonator end face where the interface state is present. Therefore, the band gap is decreased due to heat, so that a laser beam is absorbed and carriers are generated. Since these carriers generate heat, the band gap is decreased with the increase in temperature, which further causes the absorption of the laser beam. In addition, heat is generated also due to the light absorption by the end face coating film itself, resulting in a decrease in the band gap of the resonator end face.
As a result of such a positive feedback involving the light absorption and the heat generation, finally COD occurs so that the resonator end face of the semiconductor laser melts, resulting in the breakage of a mirror of the resonator. Then, a threshold current increases and optical output properties deteriorate considerably, so that a predetermined high output power cannot be obtained. Especially since an absorption coefficient of a semiconductor laser with a shorter emission wavelength tends to increase, GaInP/AlGaInP based semiconductor lasers with an emission wavelength of 630 to 680 nm are susceptible to such tendency, which hinders the realization of high output power for such semiconductor lasers.
To cope with these problems, JP11(1999)-26863 A discloses that a silicon nitride or amorphous silicon film with hydrogen added therein is formed under an end face coating film to increase the threshold at which COD results. With this configuration, the temperature rise caused by the light absorption at the end face of the laser and hydrogen supplied from the hydrogen supplying film function so that dangling bonds in the vicinity of the resonator end face terminate with hydrogen, whereby the interface state at the resonator end face of the semiconductor laser is inactivated.
JP 9(1997)-326531 A discloses, as shown in FIG. 9, a semiconductor laser 601 provided with end face coating films 602 and 603, having a configuration for improving the generation level of COD by decreasing the light absorption at the end face coating film 603. Reference numeral 601a denotes an active layer. The end face coating film 603 is made up of a laminated film of a silicon oxide film 607 and a hydrogen-added amorphous silicon film 609, i.e., a configuration in which an amorphous silicon film is substituted with the hydrogen-added amorphous silicon film 609 that has a smaller absorption coefficient.
The following describes the mounting of a high-power semiconductor laser, with reference to FIGS. 10A to 10C. In the mounting process, firstly as shown in FIG. 10A, a high-power semiconductor laser element 701 is held by a collet 704 to be mounted on a member such as a sub-mount 703 with solder 702 interposed therebetween. During this process, the sub-mount 703 is heated at a melting point of the solder 702 or higher. After the high-power semiconductor laser element 701 is lowered to the state illustrated in FIG. 10B, the sub-mount 703 and the semiconductor laser 701 are pressed and bonded with each other. After that, the collet 704 is raised as shown in FIG. 10C.
In the above-stated process, when the sub-mount 703 and the semiconductor laser 701 are pressed and bonded with each other, a residual stress tends to be generated due to a load by the collet 704 and the shape of the high-power semiconductor laser element 701. This results from a general configuration of the high-power semiconductor laser 701 in which a resonator length of 0.5 to 1 mm is relatively longer than a width. A semiconductor laser element normally is bonded with a sub-mount at a surface close to the light-emission region in order to have good heat dissipation properties, and therefore the residual stress generated inside the semiconductor laser element also concentrates on a junction with the sub-mount. Thus, a distortion due to the residual stress causes deterioration in laser properties and reliability during a long term operation.
To mitigate such residual stress, some methods are adopted in which the high-power semiconductor laser element 701 is mounted at high temperatures of 200° C. or higher, or after the mounting and under a condition without a load where the collet 704 is released, heat is applied again so as to allow the solder 702 to melt again (to 350° C.).
In this way, in order to realize a semiconductor laser with high output power and a stable operation for a long term, it is effective to suppress the light absorption by using a hydrogen-added film as an end face coating film. Also, it is effective to mitigate the residual stress by mounting a semiconductor laser at high temperatures.
However, when a high-power semiconductor laser is manufactured so as to satisfy the configuration and conditions, it has been found as shown in FIG. 11, that peeling 803 occurs in the end face coating film 802 of the semiconductor laser 801 after the mounting of the semiconductor laser, so that a resonator end face 804 is exposed. It can be considered that this phenomenon occurs due to the following reason: that is, as a result of the exposure of the semiconductor laser provided with the hydrogen-added film as the end face coating film at high temperatures of 200° C. or higher, hydrogen included in the hydrogen-added film is diffused and accumulates between the resonator end face and the end face coating film. This portion swells so that the end face coating film peels off like a blister. If the peeling of the end face coating film occurs, the laser outgoing end face is exposed to the air, and therefore this face is oxidized during a long term operation, which leads to the deterioration of the resonator end face and degrades the reliability.
Furthermore, the hydrogen-added amorphous silicon film, although intended to decrease the absorption coefficient, causes the following problems: that is, the desorption of hydrogen due to the application of heat causes a change in the refractive index of the film, resulting in an increase in the absorption coefficient as well as a change in the end face reflectance, which degrades the generation level of COD and the laser properties.